Cryo-EM structure ofex vivofibrils associated with extreme AA amyloidosis prevalence in a cat shelter

AA amyloidosis is a systemic disease characterized by deposition of misfolded serum amyloid A protein (SAA) into cross-β amyloid in multiple organs in humans and animals. AA amyloidosis occurs at high SAA serum levels during chronic inflammation. The disease can be transmitted horizontally, likely facilitated by prion-like mechanism, in captive animals leading to extreme disease prevalence, e.g. 70% in captive cheetah and 57-73% in domestic short hair (DSH) cats kept in shelters.Herein, we present the 3.3 Å cryo-EM structure of an AA amyloid extractedpost-mortemfrom the kidney of a DSH cat with renal failure. The structure reveals a cross-β architecture assembled from two 76-residue long proto-filaments. Despite >70% sequence homology to mouse and human SAA, the cat SAA variant adopts a distinct amyloid fold. Based on shared disease profiles and almost identical protein sequences, we propose a similar amyloid fold of deposits identified previously in captive cheetah.